Page 68 - Centrifugal Pumps 2E

54 Centrifugal Pumps: Design and Application

dividing rib make this type of casing very difficult to manufacture and
almost impossible to clean. In large pumps double-volute casings should
be generally used and single-volute designs should not be considered,

The Double-Volute Dividing Rib (Splitter)

The dividing rib or splitter in double-volute pumps causes considerable
problems in the production of case castings. This is particularly true on
small-capacity pumps where flow areas are small and a large unsup-
ported core is required on the outside of the dividing rib (splitter).
The original double-volute designs maintained a constant area in the
flow passage behind the splitter. This concept proved to be impractical
due to casting difficulties. In addition the consistently small flow areas
caused high friction losses in one of the volutes, which in turn produced
an uneven pressure distribution around the impeller. Most modern de-
signs have an expanding area in this flow passage ending in equal areas
on both sides of the volute rib.
The effects of volute rib length on radial thrust are shown in Figure
5-3. Note that the minimum radial thrust was achieved during Test 2 for
which the dividing rib did not extend all the way to the discharge flange.
Also note that even a short dividing rib (Test 4) produced substantially
less radial thrust than would have been obtained with a single volute.

Triple-Volute Casings

Some pumps use three volutes symmetrically spaced around the impel-
ler periphery. Total area of the three volutes is equal to that of a compara-
ble single volute. The triple volute casing is difficult to cast, almost im-
possible to clean, and expensive to produce. We do not see any
justification for using this design in commercial pumps.

Quad-Volute Casings

Approximately 15 years ago a 4-vane (quad) volute was introduced.
Later this design was applied to large primary nuclear coolant pumps
(100,000 GPM, 10-15,000 HP). The discharge liquid passage of these
pumps is similar to that of a multi-stage crossover leading to a side dis-
charge. There is no hydraulic advantage to this design.
The only advantage of this design is its reduced material cost. The
overall dimensions of quad-volute casing are considerably smaller than
those of a comparable double-volute pump.